1. Field of the Invention
The field of the invention is that of flight management systems for aircraft comprising a terrain anti-collision system and a collimated viewing device able to present symbologies dedicated to the avoidance of obstacles.
2. Description of the Prior Art
In the aeronautical sector, collisions with the ground without loss of control, commonly called CFITs, the acronym standing for “Controlled Flight Into Terrain”, are the prime cause of catastrophic accidents to civilian airplanes. The aeronautical industry is concentrating its efforts on means for reducing and ultimately eliminating all futures CFIT accidents.
For thirty years now, to solve this problem, the aeronautical industry has been developing a tool called GPWS standing for “Ground Proximity Warning System”. However, GPWS has no systems for recognizing the situation of the terrain and does not know the relative trajectory of the airplane with respect to the terrain. The ever growing increase in air traffic has given rise to a new growth of CFITs despite the quasi-generalized use of GPWS. Today, CFITs still represent more than 40% of accidents.
The technological advances obtained notably in the development of digitized terrain files, in the precision of positioning by virtue of GPS location and in increased processing power have allowed the development of new concepts making it possible to guard against these risks of collision with the ground. These concepts rely essentially on extrapolation of the current trajectory of the airplane and on a terrain database corresponding to the relief overflown making it possible to predict these risks. These concepts are materialized in the form of a new generation of equipment intended for transport or business planes called TAWS for Terrain Awareness and Warning System. This system is, for example, marketed by the company THALES AVIONICS under the brand name GCAM standing for Ground Collision Avoidance Module.
The GCAM system is described in FIG. 1. It essentially comprises a central electronic computer linked on the one hand to the network of sensors and pick-ups of the aircraft and on the other hand, to the various displays of the instrument panel as well as to the audible alarms disposed in the cockpit by means of a data transmission bus. The sensors are essentially sensors making it possible to determine the position of the craft with respect to the ground, its attitude and its speed. The displays concerned are essentially the Head-Up sight as well as the Head-Down piloting and navigation screens such as the “Navigation Display” and the “Primary Flight Displays”.
The dynamic operation of the GCAM is as follows. It monitors the geographical environment of the aircraft. If it finds that there is no risk of terrain threat, near or far, the depictions presented to the pilot and optionally to the copilot are standard for a mission of IFR type, the acronym standing for Instrument Flight Rules or VFR type, the acronym standing for Visual Flight Rules. The system is in a mode termed “NORMAL”.
When the system determines that there is a possibility of dangerous terrain along the axis of the aircraft and/or laterally, the system passes to a mode termed “LATERAL PROXIMITY”. In this mode, the depictions must allow the pilot to carry out the navigation tasks and to comprehend the situation without ambiguity. The distance from the aircraft to the obstacles is of the order of forty Nautical Miles.
In this case, if the pilot does not react, the airplane approaches dangerously close to the terrain, its trajectory touching the relief. As soon as the time before the collision becomes less than about twelve seconds, the system passes to a mode termed “CAUTION”. The audible alarm “TERRAIN TERRAIN” sounds. Alarm messages are also displayed on the screens.
The pilot has understood the situation. He must now act by piloting manually. When he has only about eight seconds left to perform a correct avoidance maneuver, the system passes to a mode termed “WARNING”. The proposed avoidance maneuver is either vertical of the type “PULL-UP” or involves a turn of the type “PULL UP-TURN RIGHT” or “PULL UP-TURN LEFT”. The audible alarms sound. The messages are also displayed on the screens.
The situation reverts to usual. The obstacle is avoided. The system passes back to the “NORMAL” mode or to the “LATERAL PROXIMITY” mode if it remains in the vicinity of potentially dangerous terrains. The audible alarm “CLEAR OF TERRAIN” sounds. This return to normal is accompanied by a return to a conventional display in the customary operating modes.
The GCAM allows notably the generation of a specific audible alarm or “warning” called “Avoid Terrain” in addition to the conventional “warning” called “Pull Up” which corresponds to a vertical avoidance maneuver. This alarm is engaged when an avoidance maneuver by “Pull-Up” no longer makes it possible to ensure a clearance without collision. The “Avoid Terrain” alarm of the GCAM, though representing a significant advance, does not completely meet pilot expectations. They would like to have an indication of lateral avoidance maneuver when no vertical avoidance maneuver is possible any longer.
During the avoidance maneuvers, the pilot must essentially navigate and pilot his machine manually in the vertical and horizontal planes. These tasks are detailed below:                Navigating:                    Identifying/Grading the dangerous obstacles;            Analyzing the situation to protect oneself from threats;            Comprehending the new trajectories proposed by the system.                        Piloting the machine in the vertical plane:                    Maintaining attention on the current vertical speed;            Maintaining attention on the current altitude;            Maintaining attention on the proximity of the ground;            Being aware of the attitude of the airplane with respect to the real world;            Maintaining attention on the speed vector of the airplane;            Maintaining attention on the angle of attack so as to avoid stalling;            Maintaining a climb slope.                        Piloting the machine in the horizontal plane:                    Maintaining attention on the current course followed by the airplane;            Maintaining attention on the current roll of the airplane;            Entering a turn correctly;            Holding a turn correctly;            Exiting a turn correctly;            Maintaining a correct trajectory with respect to a predefined avoidance trajectory.                        
The instrument panels of modern aircraft comprise so-called Head-Down piloting and navigation display screens such as the “Navigation Display” and the “Primary Flight Display”. An exemplary symbology present on a “Primary Flight Display” is represented in FIG. 2. It conventionally comprises:                in its upper part, an aircraft attitude indicator 100 also called the “artificial horizon” surrounded by the air-speed indicator 300, vertical speed indicator and altitude indicator 200;        in its lower part, a heading rose 400 with the conventional indications for course and course deviations.        